Gamma-Ray Bursts in the Multi-messenger Era

Despite the recent progress in Gamma-Ray Burst science, obtained in particular thanks to the Swift and Fermi satellites, there are still many open questions in the field. One concerns the mechanisms that power these extreme explosions, which are still unclear after more than four decades since their discovery. In particular the content of the relativistic flow that produce the GRBs remains to be investigated: especially in terms of its geometry, bulk Lorentz factor, magnetization, and baryon loading, and internal dissipation mechanisms. The nature of GRB progenitors is also debated. While there is a consensus on progenitors of long GRBs, as being very massive stars, the situation is less clear for what concerns the short GRBs: the most popular models involve the possibility of a coalescence of two compact objects (NS+NS/NS+BH), but a direct proof of this model is still lacking. In addition, an important issue is the possibility for GRBs to be the source of Ultra-High Energy Cosmic-Rays (UHECR).

All these questions can be tackled with the use of upcoming neutrino and gravitational waves facilities, the so-called new-messengers. First neutrino candidates from a cosmic source have been recently detected by IceCube but they could not be correlated with any precise astrophysical source. While no direct observation of gravitational waves has yet been claimed, it is widely believed that a first detection could plausibly occur in the near future. GRBs are among electromagnetic counterpart candidates, and a simultaneous detection would represent a major milestones bridging neutrino and gravitational wave observations with conventional astronomy. Neutrinos could play an important role in understanding the mechanisms of cosmic-ray acceleration, and their detection from a cosmic source would be a direct evidence of the presence of hadronic acceleration. The production of neutrinos of 100 TeV then necessitates the acceleration of protons up to PeV energies and is therefore expected independently of the question to know if GRBs are the source of UHECRs. Depending on the details of the model considered, these high-energy neutrinos are emitted in coincidence with, or as a precursor signal to gamma-ray emission.

A new generation of gravitational wave detectors, Advanced LIGO, Advanced Virgo and Kagra, are currently under construction and will reach their design sensitivity around 2018-2020. Thanks to the ten-fold improvement in sensitivity with respect to the previous generation, these detectors are likely to make the first direct detection of gravitational waves. GRBs are believed to be an important source of gravitational waves, since post-Newtonian theory predicts a distinctive gravitational-wave chirp signal from the inspiral stage of NS-NS or NS-BH coalescences, so that the detection of such a signal associated with a short GRB would provide “smoking gun” evidence for the binary nature of the GRB progenitor. In addition recent studies are also focusing on the possibility of GW emission from long GRB.